Shuffling the Spectrum Deck

Government seeks novel methods to squeeze the most from crowded radio frequency environment.

Future military communications equipment may one day be able to detect and use locally available radio spectrum automatically. U.S. Defense Department researchers are developing methods that allow systems to scan for unassigned frequency bands autonomously. These technologies will allow warfighters to deploy quickly anywhere in the world without time-consuming spectrum management and allocation concerns.

The airwaves are a crowded place. One of the major challenges the U.S. military will face in this century is how to use shrinking bandwidth efficiently and provide a viable solution to meet commercial carriers’ needs for additional spectrum, U.S. Air Force officials say. The limits of current military spectrum allocations for domestic and foreign uses will become strained because of growing commercial and government needs.

Demand for higher capacity and more complex data services such as assured communications is predicted to increase by some 250 percent in the next decade, the officials observe. This will be coupled with the explosive growth of the commercial wireless market, which is currently pressing the government for an additional 100 megahertz of Defense Department assigned spectrum for use with third-generation cellular technology (SIGNAL, December 2001, page 49).  The military also is shifting to new warfighting technologies that provide a high degree of situational awareness and to new doctrines that emphasize connectivity for coordination.

The Air Force estimates that Defense Department emitters reuse only about one percent of available spectrum. Inefficient use by legacy communications systems and allocation procedures contribute to these low utilization rates, officials say. Because of inconsistencies in the international spectrum allocation environment, current and projected garrison-level issues are compounded when forces deploy overseas. Dynamic spectrum allocation may solve this potential crisis for U.S. military and government systems. However, to fully exploit and control this technology, it will be necessary to develop capabilities beyond commercial third-generation wireless systems, officials say.

Because network-centric warfare depends on continuous data feeds, the Defense Department is exploring ways to ensure that the necessary bandwidth is always available anywhere in the world. This is the goal of the Next Generation (XG) Communications program, a cooperative venture between the Defense Advanced Research Projects Agency (DARPA), Arlington, Virginia, and the U.S. Air Force Research Laboratory, Rome, New York.

According to Dr. Paul Kolodzy, program manager of DARPA’s Advanced Technology Office, military spectrum managers must extensively plan and assign frequency and prevent any conflicts during an operation. This time-consuming process can take weeks or months of preparation before a deployment, and once assigned, equipment is rigidly fixed to that specific spectrum. He notes that current systems and frequency management plans dictate these static assignments to users whose operations and spectrum demands are dynamic by nature. The XG program seeks to develop both the enabling technologies and concepts to dynamically utilize spectrum.

Dynamic spectrum management is vital because information dominance is rapidly becoming a key factor in warfighting doctrine. “Future concepts will compete for resources in military spectrum bands that are already near capacity under current spectrum management capabilities. If advanced warfighting concepts are to be realized and their communications needs supported, they will require the increased spectrum efficiency and agility under development in the XG program,” Kolodzy says.

Current warfighting plans call for assigning spectrum with 99.9 percent reliability and a connection success rate of 97 percent. This static, risk-averse approach can take months to plan and deconflict prior to a deployment, Kolodzy explains. However, allocation is not distributed evenly because some frequencies are used continuously while others are hardly tapped at all. Dynamic spectrum use would maximize efficiency by exploiting unused “holes” in the spectrum that may last for several milliseconds. Static systems, by comparison, interrupt transmissions for priority messages and react to spectrum changes in seconds or milliseconds, depending on spectrum availability.

Through the program, DARPA seeks to create and demonstrate a range of innovative approaches to improve military radio frequency (RF) emitters by a factor of 20. The effort will lay the theoretical groundwork for dynamic spectrum control and develop system applique prototypes to prove the technology’s applicability in legacy and future RF emitters.

Another facet of this approach is the construction of an integrated system by utilizing microelectronics, new waveform and medium access and control (MAC) protocol technologies. Other programs may be leveraged to contribute research to the XG program. They include work such as nano-mechanical-array signal processing designed to create precision nano-scale structures for RF-signal processing, and intelligent RF front-ends demonstrating a new generation of tunable analog/RF components capable of self-assessment and adaptation to the spectrum environment. In addition, research with antimonide-based-compound semiconductors designed for low power consumption and adaptive command, control, communications, computers, intelligence, surveillance and reconnaissance (C4ISR) nodes could provide simultaneous signals intelligence and communications.

Research for the communications component of the future combat systems program could aid the XG effort by contributing work on high- and low-band directional antennas; advanced modulation, coding and signal processing in a software radio framework; RF information assurance; and mobile networking with directional antennas for real-time traffic.

Another DARPA program whose technology could aid spectrum management efforts is the small unit operations situation awareness system (SUO SAS) (SIGNAL June 2001, page 21). The SUO SAS researchers are developing small, adaptive multifunctional communications systems for individual soldiers. The devices will possess autonomous adaptive networking, position location/navigation and distributed information management capabilities.

The XG effort will seek to develop, integrate and evaluate these methods, enabling equipment to select spectrum and operating modes automatically while minimizing disruption to existing users and ensuring the operation of U.S. systems. The final result of the program will be to develop and demonstrate an appliqué for future and legacy emitter systems for joint service use, DARPA officials maintain.

The program also will develop enabling technologies and system concepts to provide assured military communications that support global, short-notice deployments by dynamically redistributing allocated spectrum. According to DARPA, this research will significantly affect a variety of current and future communications systems in government and commercial use. Systems that will be specifically affected are radar, ground and air tactical communications, unmanned/robotic vehicle operations, data/video links and commercial mobile wireless operations.

The technology provides a common technical architecture to meet both future military and civilian mobile communications needs. This effort will require the involvement of a variety of government and commercial research and development centers, the support of service operational communities and government spectrum agencies such as the Federal Communications Commission and the Office of Spectrum Analysis and Management, DARPA officials say.

One thrust of the program will be to develop an appliqué that can automatically exploit unused spectrum without interfering with assigned users and legacy communications systems. These systems and programs must allow XG equipment to sense and characterize the spectrum environment quickly, then react and adapt to changes while maintaining network connectivity and avoiding degradation in legacy system operations. This is the main principle behind dynamic spectrum allocation, Kolodzy explains. He notes that current dynamic spectrum utilization systems typically employ dedicated frequency bands. In contrast, XG technologies will opportunistically access available spectrum across a number of bands on a noninterference basis without requiring an exclusively dedicated spectrum band.

The program will be implemented in three phases. Phase one will run through 2003 and determine the viability of XG concepts by investing in spectrum measurements and initial technology development. If these efforts prove successful, the program will culminate with an end-to-end mobile systems demonstration in 2006. Kolodzy notes that the program recently reviewed the first round of industry white papers submitted for phase one.

The XG program is currently focusing on mobile terrestrial applications for military communications systems. “Mobile military bands are seeing increased demand as more information is channeled to individual warfighters. Finding available spectrum resources in those bands is one of the biggest challenges to spectrum managers,” Kolodzy says.

Researchers will face additional challenges in developing systems that can sense, characterize, react and adapt to spectrum environments in tens or hundreds of milliseconds while maintaining network connectivity. He explains that at this stage of the program, the high-risk technologies focus on this ability to react and adapt quickly and automatically to spectrum changes from a time, frequency and space perspective.

Advances gleaned from the XG program may ultimately be applied to or supercede efforts such as the joint tactical radio system (JTRS). The JTRS is designed to provide the military with a software programmable tactical radio capability that allows a single system to be reconfigured to meet communications plans and needs. However, the XG concept will not only be software-based and configured as needed to comply with local regulations and constraints. It also will be able to access available spectrum dynamically within those constraining parameters, Kolodzy maintains.